Whether you're alone or have an extra set of hands, there are a couple of key things you need to do when spooling a fishing reel. We'll get those out of the way first and then dive into a few ways to make spooling a reel by yourself a bunch easier.
First, almost all fishing line has some sort of memory. It may be minimal, like with braided line and some higher-end fluorocarbon, but if you just let the line fall off the spool, you'll see some curl to it.
So you need to be certain that the line is coming off the spool and going onto the reel in the same direction. If you don't, you'll be fighting that memory when you cast your bait and find yourself backlashing and battling line twists far more often.
If you're holding a baitcaster in your hand with the opening of the reel facing upward, the line will come into the reel, across and then under the spool as you turn the handle. So you want to be sure your filler spool is rotating in the same direction. If it is, the line will be coming off the top of the filler spool and not the bottom.
For a spinning reel, the same principal applies. Ironically, however, the spool of a spinning reel doesn't spin. So you don't want your filler spool to spin either. Instead, you want to turn the filler spool so you're facing the flat side of it and the line is hanging down from the right side of the spool. You'll see then that the line is coming onto your reel the same way it's coming off of the spool.
The next thing you want to be certain to do with both baitcasters and spinning reels, is keep a little tension on the line as it is spooled onto the reel. The easiest way to do this is by sliding your non-dominant hand (left for right-handed reels, right for left-handed reels) up to the section of the rod just in front of the reel. Use your thumb and forefinger to pinch the line and provide tension as it enters the reel.
This will actually help take some of the memory out of line and make sure that the line pulls down tight to the reel as you spool it on. This is very important for minimizing backlashes. It will also help prevent your line from cinching down deep into the spool when you break off or do get a bad backlash. If the line closer to the spool isn't wound on tight, the line above will cinch down into itselft when pulled hard.
Now that we've set the scene for properly spooling a reel, how do you do it by yourself?